I have been offered by several of my debtors/collection agencies a settlement of 50%, my question is most of my debt is now interest. If the company that is hired purchased my debt or was assigned to collect my debt do I negotiate a lower settlement? I have heard these companies buy delinquent debt for penny's on the dollar, how do I properly negotiate?

You don't, they don't have to offer you anything, if you owed Visa (for example) $2000 and half of that was interest, Visa was allowed to collect $2000.00 and so is the collection agency. They can always say never mind we want the whole amount.

They are probably doing this since the amount is not high enough to consider suing you and they just want it closed, it's probably something you should consider.

I would counter offer them about 25 percent and see what happens.
But do it nicely.